In the present specification and claims, the term "sorting" is used to mean categorization of articles by size and/or color and the term "grading" is used to mean categorization of articles by quality and/or type.
Systems for sorting and grading of apples based on computing thinness ratios for blemishes are described in the following two publications by Rehkugler, G. E. and Throop, J. A.:
"Image processing algorithm for apple defect detection", Trans. of the ASAE, Vol. 32(1), pp. 267-272, 1989, and PA1 "Apple sorting with machine vision", Trans. of the ASAE, Vol. 29(5), 1388-1397, 1986. In this reference, diffuse illumination is provided by a translucent acrylic box.
A Ph.D. dissertation by G. L. Graf ("Automatic detection of surface blemishes on apples using digital image processing", Cornell University, 1982, available from UMI Dissertation Information Service, 300 N. Zeeb Rd. Ann Arbor, Michigan 48106) discloses in FIG. 4--4 a generally cylindrical diffuser and apparatus for illuminating the diffuser from the exterior thereof, for inspecting an apple which has been manually placed within the cylindrical diffuser.
Another cylindrical diffused light system is described in Sarker, N. and R. R. Wolfe, "Computer vision based system for quality separation of fresh market tomatoes", Trans. ASAE, Vol. 28, pp. 1714-1718, 1985. The Sarker et al system includes mechanical apparatus for aligning tomatoes along the stem/calyx axis and a defect detection subsystem which employs adaptive thresholding of a gradient image.
Color inspection methods for peaches in which peaches are viewed from more than one angle using multiple sensors, a frame grabber, a CCD camera, one-sided diffuse illumination and color grading and feature extraction algorithms are mentioned in Miller, B. K. and Delwiche, M. J., "A color vision system for peach grading", Trans. of the ASAE, Vol. 32(4), 1484-1490, 1989.
An EO commercial development of a sorter and grader system employing solid-state CCD cameras in which blemishes are detected on fruit rolling on bicone rollers is described in A. Davenel et al, "Automatic detection of surface defects on fruit by using a vision system", J Agricultural and Engineering Research, 41, 1-9, 1988.
Methods for identifying bruised apple tissue using reflected light of one, two or three normalized wavelengths, including computations of differences or ratios between 2 wavelengths, are discussed in B. L. Upchurch et al, "Spectrophotometric study of bruises on whole, red delicious apples", Transactions of the ASAE, 33(2), Mar.-Apr. 1990.
Upchurch et al describe various models for distinguishing bruised from nonbruised areas on Red Delicious apples. One such model is set forth in Equation 5 and comprises a two wavelength such model is set forth in Equation 5 and comprises a two wavelength difference which is divided by a normalizing wavelength to take into account illumination changes. Upchurch et al propose to cancel out differences in reflectance levels, due to factors other than tissue condition, by division.
U.S. Pat. No. 5,085,325 to Jones et al discusses color sorting of articles and use of a camera and flash to image moving articles. Color analysis is performed on the basis of RGB color imaging and spherical color coordinates.
U.S. Pat. No. 5,012,116 to Russell discusses a system for detecting defects in diffusely illuminated bearing balls which roll along a pair of parallel rails.
U.S. Pat. No. 5,010,247 to Smith et al discloses a system in which objects are dropped through a viewing zone and are viewed by three viewers arranged along 3 mutually orthogonal axes using radiation of different wavelengths.
A method for deriving information regarding convexity/concavity of an article by projecting a grid pattern on the article is disclosed in N. Shrikhande and G. Stockman, "Surface orientation from a projected grid", IEEE Transactions on pattern analysis and machine intelligence, Vol. 11(6), June 1989.
Attempts to detect fruit stems using image processing are reported in Wolf, R. R and Sander, W. E, "An algorithm for stem detection using digital image processing", Trans. ASAE, 28, pp. 641-644, 1984.
U.S. Pat. No. 4,863,041 to Bailey discloses a system for observing an article from 4 sides without imaging, using a color background which is dynamically controlled. The system is triggered as a response to intercepted light beams.
U.S. Pat. No. 5,078,258 to Van Der Shoot discloses a system which orients apples by mechanical apparatus and subsequently picks the apple. The system includes a color camera which identifies "red cheek" on an apple.
U.S. Pat. No. 4,940,850 to Satake discloses a system of 3 detectors which "look" at an article in vertical free fall through 2 spectral filters.
U.S. Pat. No. 4,878,582 to Codding discloses a bichromatic sorter in which an article is seen from 3 coplanar directions, on a color background.
U.S. Pat. No. 4,741,042 to Throop et al discloses a software system for analyzing size and shape of bruises on fruit by creating a binary image of bruises.
U.S. Pat. No. 4,645,080 to Scopatz discusses a system for grading non-orienting articles. Specifically, the system grades oranges on the basis of information from three overhead and side sensors.
U.S. Pat. No.4,515,275 to Mills et al discloses a system for processing fruit such as lemons. Polarized illumination of the fruit is provided from 4 sides and a single video camera or line scanner is employed.
U.S. Pat. No. 4,204,950 to Burford, Jr. discloses a grading system using four spectral bands, of which two are visible.
A state of the art system for conveying fruit is marketed by Accu Pak Systems.
Published PCT Application WO 91/04803 describes apparatus for weighing, sizing and defect sorting of fruit including one or more singulators, a conveyor for passing the fruit under a CCD array camera, means for rotating the fruit under the camera, an image processor including a master processor passing signals to a vision processr and 8 object processors which divide the captured image into image sections each representing only one piece of fruit, and ejectors spaced along the conveyor which eject the fruit to outfeed conveyors corresponding to sorting categories.